A DC-DC voltage converter can convert a continuous input voltage, which may be a voltage from a continuous voltage source, into a continuous output voltage via an inductor between the input and the output as reactive component. Since the voltage source may be a battery, which discharges over time, the converter may sometimes also be arranged to guarantee an output voltage at a determined value independently of a drop in input voltage.
Various types of inductive DC-DC converters may be used. A first type of DC-DC converter is the Buck converter which can supply an output voltage of lower value than the input voltage value. A second type of DC-DC converter is the Boost converter which can supply an output voltage of higher value than the input voltage value. A third type of DC-DC converter can combine a Buck converter and a Boost converter to raise or lower the output voltage.
FIG. 1 shows an inductive DC-DC converter, which is a Boost converter generally used in low power electronic circuits, powered by a battery of small dimensions. The DC-DC converter includes an inductor L as reactive component, a PMOS transistor P1 and an NMOS transistor N1, which act as switches. PMOS transistor P1 and NMOS transistor N1 are series connected. Inductor L is disposed between a positive terminal Vbat of a continuous voltage source Bat, and a connection node of the drain terminals of transistors P1 and N1. The source terminal of transistor N1 is connected to an earth terminal, whereas the source terminal of transistor P1 is connected to an output terminal Vout, where filtering is performed by the capacitor C connected in parallel to a charge resistor RL.
Transistors P1 and N1 are operated alternately, each by a respective control signal CK1 and CK2 across their respective gate terminal. Control signal CK1 defines a first converter control phase, whereas control signal CK2 defines a second converter control phase. During a first determined time period, control signal CK1 is at the high state to make NMOS transistor N1 conductive. During a second determined time period subsequent to the first time period, control signal CK2 is at the low state to make PMOS transistor P1 conductive.
During the first time period, the current in inductor L increases linearly, whereas during the second time period, the current in inductor L decreases linearly, ideally to reach zero. If, at the end of the second time period, the current through the inductor is not zero, an adjustment must be made to the second time period. This adjustment of the second time period is generally necessary for a discontinuous mode DC-DC converter. It sometimes requires at least one fast comparator, which causes high power consumption.
EP Patent No 2 104 213 B1 discloses a control method for a discontinuous mode DC-DC converter. The duration of the second time period, during which the current in the inductor decreases, is controlled so that the current is zero at the and of the second time period. The sign of overvoltage caused by the inductor during switching at the end of the second time period makes it possible to adjust the duration of the second time period. However, this principle of adjustment of the second time period by controlling overvoltage requires the use of fast electronic components, which causes high power consumption thereby constituting a drawback.